An electrophotographic offset printing plate (hereinafter referred to as the "master") has a light-sensitive layer comprising fine particles of a photoconductive material (e.g., zinc oxide) dispersed in a resin binder; this plate is subjected to a conventional electrophotographic operation for forming an oleophilic image on the light-sensitive layer.
Offset printing generally uses a plate comprising a highly water wettable non-image area (hydrophilic area) and a poorly water wettable image area (oleophilic area). However, the master for use in electrophotographic offset printing has its image area composed of a hydrophobic photoconductive layer, so if the plate is immediately subjected to printing, the printing ink adheres to the non-image area and prevents normal operation. It is therefore necessary to render the non-image area of the master hydrophilic by desensitizing it before printing. Desensitizing solutions (also called "etching solution") that have been proposed to date are classified in two types, one type being a cyan compound containing solution mainly comprising a ferrocyanate (as described in U.S. Pat. No. 3,001,872) or ferricyanate, and the other type being a cyan free solution mainly comprising an amine cobalt complex (as described in U.S. Pat. No. 4,208,212), phytic acid (as described in U.S. Pat. No. 3,592,640 and Japenese Patent Publication No. 2839/83) or its derivative, or a guanidine derivative.
However, none of these processing solutions are completely satisfactory. The first type of solution containing a ferrocyanate or ferricyanate has the advantages of high desensitizing power, ability to form a strong hydrophilic film, and fast film formation. Nevertheless, the ferrocyanide or ferricyanide ion is unstable to heat and light, and when exposed to light, it undergoes either a color forming reaction or a precipitation reaction that weakens the desensitizing power of the solution. Furthermore, the presence of cyanide ions (CN.sup.-) leads to the formation of a detectable free cyanide which is carried into the effluent and pollutes water-courses.
The cyan-free desensitizing solution has been proposed to eliminate these defects of the cyan-containing solution. But even this second type of desensitizing solution has been unable to provide a completely satisfactory lithographic printing master. Particularly, the cyan-free solution has a slower rate of hydrophilic film formation than the cyan compound-containing solution, and is unable to form an immediately printable, hydrophilic film of high physical strength by one pass through an etch processor, and as a result, scum often forms on the non-image area, or insufficient edge acuity of halftone dots occurs.
Inositol hexaphosphate esters and metal derivatives thereof are known to form chelate compounds with metals, and have already been proposed for use as desensitizers in the processing of the offset printing master as described in U.S. Pat. No. 3,592,640. However, these compounds have such a slow film forming rate that the desired hydrophilic film is not obtainable by a single pass through the processor, and the resulting poor ink separation may often cause scum on the non-image area or insufficient edge acuity of halftone dots.
In previously filed Japenese Patent Publication No. 5799/83, use has been proposed of an ion complex of a water-soluble cationic polymer and a mono- or divalent metal salt of an inositol hexaphosphate ester (e.g., sodium phytate, potassium phytate or calcium phytate). This complex has improved water retention but does not achieve a satisfactory etching speed. At the same time, the complex is highly dependent on moisture and is so sensitive to the printing environment that scum easily forms on the non-image area during printing at low humidity.